The present invention relates generally to an optical system with a phase type Fresnel zone plate incorporated in it, and more particularly, to an optical system making use of a phase type Fresnel zone plate having so improved a structure that it can work better and be made more easily than ever before.
Heretofore, the vitreous material available for projecting lenses with demagnification incorporated in stepping projection aligners using KrF excimer laser as light sources has been substantially limited to quartz glass alone. This has rendered it unfeasible to correct chromatic aberration by the combined use of vitreous materials differing in dispersion. Thus, methods of using phase type Fresnel zone plates to correct chromatic aberration have been proposed in the art.
The principle of refraction of a ray by a phase type Fresnel zone plate--it is here noted that although the term "refraction" should be described in terms of diffraction or bending, it will be used elsewhere in the present disclosure, unless otherwise stated, because the phase type Fresnel zone plate hereinafter referred to may be deemed as a convergent or divergent optical element equivalent to a refracting lens--is the same as that of diffraction of a ray by a diffraction grating, and the angles .theta. and .theta.' of the ray incident on and leaving the normal of the phase type Fresnel zone plate may be expressed by the Fresnel pitch p (the distance between adjacent zonal areas) corresponding to the grating pitch of the diffraction grating and the wavelength .lambda. of the ray, i.e., EQU sin.theta.'-sin.theta.=m.lambda./p. (1)
Here, m stands for the order of diffraction say, 0, .+-.1, .+-.2, .+-.3, .+-.4, . . .
Equation (1) teaches that the amount of refraction of a ray increases with a wavelength increase. To put it another way, this indicates that the refractive power (the reciprocal of the focal length) of a phase type Fresnel zone plate increases with a wavelength increase. Since generally available optical glass materials inclusive of quartz glass have positive dispersion, on the other hand, lenses made up of such optical glass materials decrease in their refracting powers with wavelength increases. In other words; the phase type Fresnel zone plate has a lens action, but that lens action is apparently tantamount to that of a lens made up of an optical material having negative dispersion. If materials differing in dispersion are used, then chromatic aberration may be compensated for. For instance, Japanese Provisional Patent Publication No. Hei. 2-1109 (corresponding to U.S. Pat. No. 4,936,665) sets forth a technique, according to which a phase type Fresnel zone plate is placed at the pupil location of a projecting lens with demagnification incorporated in a stepping projection aligner using KrF excimer laser as a light source, thereby correcting chromatic aberration. The phase type Fresnel zone plate disclosed therein has a positive refractive power making use of a single order of diffraction. The use of a single order of diffraction makes it difficult to achieve high diffraction efficiency by diffraction of higher order. Low diffraction efficiency gives rise to harmful rays other than the light subject to the desired refraction, which are otherwise responsible for low image surface contrasts. Thus, there is no choice but to make use of diffracted light of low order, say, first or second order.
In this connection, it is noted that since the ability of a phase type Fresnel zone plate to correct chromatic aberration is determined by its refractive power, it must have a given refractive power so as to correct chromatic aberration well enough. On the other hand, the pitch of the phase type Fresnel zone plate decreases gradually from the optical axis in the radial direction, but the more the refractive power, the smaller the minimum Fresnel pitch, as will be understood from Equation (1). This makes plate production difficult. To correct chromatic aberration well enough, the minimum Fresnel pitch must be as fine as 5 .mu.m or below. This makes plate production all the more difficult.